The present invention relates to the field of prosthetics, and, in particular, to a biomechanical system designed to assist an amputee in walking, running and other types of locomotion.
I propose to apply a biomechanical emulation of metatarsal and ankle joint movement by incorporation of the necessary mechanical features in the design of the artificial foot and ankle. By mimicking ankle and metatarsal joint movement in the artificial foot, a more normal gait should result. A more natural sequence of the artificial joint's fixation and mobility is proposed.
Preliminary feasibility studies (Pitkin, M. R., Mendelevich, I.A.: "A Prototype of the Rolling Joints Prosthetic Foot", Proc. of the Seventh World Congress of ISPO, Chicago, Ill., Jun. 28-Jul. 3, 1992, p. 134.) indicate that non congruent rolling joint surfaces combined with and held together by linear elastic springs can mimic actual joint motion and provide nonlinear saturation in response to that motion. This investigation was based on the practical evaluation of our previous design (Pitkin M. R., Mendelevich I.A.: "Artificial Foot", Author's Certificate of the USSR No. 820,222, 1981), in which these features have been disclosed. However, the artificial foot in this patent does not provide cam rolling in the ankle zone. It also does not provide sufficient nonlinear saturation of the cam structure, since the points of attachment of the springs in the joint are not movable. In addition, the optimal timing and limits of the fixation/mobility of the prosthetic foot and ankle have not been provided.
In this application, I am using information acquired from the following study by M. R. Pitkin, "New Stride Phases and the Development of Sport Shoe Prototype to Assist the Calf Muscles During Heel-Off", Proc. The 15th Annual Meeting of the American Society of Biomechanics, Tempe, Ariz., 1991, pp. 266-267. This study shows the advantages of optimal timing and "free mobility" intervals in the ankle and metatarsal joints. In this study it was found that during the support phase of the walking stride, after sufficient and almost free mobility of the ankle joint, the calf muscles provide almost total fixation of the ankle joint. Therefore the heel lifts off and the foot begins a rotation around the metatarsal heads. Such rotation is a result of inertia of the body, and the calf muscles lift the heel indirectly by fixation of the ankle.
It is possible to describe the foot-ankle in gait using the following phases:
1) Ankle-Phase (AP); PA1 2) Metatarsal-Phase (MP); PA1 3) Metatarsal-Ankle-Hip-Ankle.sub.2 -Phase (MAHA.sub.2 P), where the symbol A.sub.2 relates to the ankle of second leg.
The beginning of AP is defined as the moment of toes-off of the rear leg.
The MP starts at the moment of heel-off.
MAHA.sub.2 P starts from the heel-on and amortization-like, or dampening, plantar flexion of the second leg, which is swinging above the ground during AP and MP. This plantar flexion continues until the foot is positioned on the walking surface.
In each of these phases a human body is representable by a system of one degree of freedom.
It was also noticed that electromyogram studies (EMG) of musculus gastrocnemius during a stance period of locomotion (when the foot is in contact with the ground) demonstrated three specific zones, correlated with the phases discussed.
The major activity of musculus gastrocnemius takes place at the MP (heel-off). In this phase the ankle angle remains almost unchanged, which indicates a fixation of the joint's mobility. This fact supports the conclusion that lifting of the heel is not by direct muscle-driven plantar flexion, but rather it is a consequence of the inertia of the body. When the second leg touches the ground, phase MAHA.sub.2 P starts, and the calf muscles of the first leg become involved in a propulsive plantar flexion (a component of the propulsion of the body).
EMG activity of musculus gastrocnemius during MAHA.sub.2 P is significantly less than during the MP. It indicates that the muscle deficit of an amputee can be effectively compensated for by the precise timing and positioning of the passive mechanical resistors.
Firstly, a prosthetic foot has to provide as total a fixation (stopping of mobility) as possible in the ankle joint from the end of the AP and through the MP.
The low level of the EMG of the musculus gastrocnemius during the (AP) supports a second requirement for the prosthetic foot and ankle to provide as free as possible mobility in the ankle at this phase.
There is another aspect of the necessity for free mobility in the ankle. In passing from AP to MP during walking with the prosthesis, a patient's stump produces the pair of forces F,-F by its proximal and distal areas. These forces act on the stump's socket and provide the moment about the point of pressure in the metatarsal joint. That moment should be no less than the internal torque, produced by the fore part of the prosthetic foot. The greater the internal torque of resistance, the greater the moment (pair of forces F,-F) produced by the patient's stump. Due to Newton's third law, the forces of the same magnitude act on the stump from the socket. Hence, the greater the internal torque of resistance, the greater the pressure that will be applied to the patient's stump. Clearly, any mechanical means which will decrease such pressure to a stump is highly desirable.
Thus, I conclude that a prosthetic foot has to provide as free a sagittal articulation as possible in the ankle zone during AP and in the metatarsal zone during MP, and as total a fixation as possible in ankle joint from the end of the AP and through the MP.